618 Vegetable Physiology. 



distributed to every tissue to which it has access, as well as to the leaves ; 

 which last, from their perspiring functions, attract a very large share, and 

 cause special ducts to be formed for their supply, which ducts are as- 

 cending, not descending, tubes. For, if we can conceive that any fibrous or 

 tubular constituents of the stem have a downward rather than an upward 

 developement, we reverse the very principle of aerial growth, and deny the 

 result of every experiment which has been made to ascertain such processes 

 of vegetable accretion. Our anonymous author, indeed, is too complaisant in 

 adopting the opinions of those who fancy they can see buds, leaves, and even 

 grafts, " sending down" vessels or fibres into the stem on which they grow, or 

 into the stock on which they are placed. He guards himself, however, in a 

 general way, against giving full credence to descriptions of whatever is said to 

 take place in the interior of a stem, movements which never have been, nor 

 possibly can be, seen ; and yet he is sometimes misled by such representations. 

 As one instance, his 140th paragraph may be transcribed, viz. " It is in the 

 vessels and woody tubes of the alburnum that the fluid absorbed by the roots 

 is transmitted to the opposite extremity of the stem, and these vessels com- 

 municate with those of the leaves, which receive it from them. In the liber, 

 on the other hand, the fluid which has been converted in the leaves into 

 nutritious sap descends again through the trunk for the purpose of nourishing 

 its diflferent parts. Of this descending sap a part is carried inwards by the 

 medullary rays, which thus diffuse it through the whole stem, as also through 

 the substance of the roots, down which it is conveyed by their bark. In this 

 descent it mixes with the ascending current, especially at its lower part ; and, 

 being much superior in density, it adds to the density of that fluid, and thus 

 maintains the condition requisite for endosmose" (that principle by which the 

 watery fluids in the soil are attracted through the spongioles of the roots to 

 mix with the thicker fluids in the stem). " The vessels," he continues, " down 

 which the sap moves in the bark are of the branching character described as 

 peculiar to those which convey the nutritious fluid. They form a complete 

 network, in which the fluid may be seen to move in various directions. For 

 this motion no definite cause can be assigned. It does not depend on any 

 impulse from above, corresponding to that action of the roots which raises 

 sap in the stem, for there is no power in the leaves to give any such force. It 

 has been supposed to depend upon the gravity of the fluid, which will cause it 

 to descend simply by its own weight; but, if that were the case, it would not 

 ascend, as it often does, on the bark of the hanging branches of such trees as 

 the weeping ash or willow. It is only one, however, of numerous cases in 

 which a movement of nutritious fluid through channels in the solid parts it 

 supplies takes place without any evident cause." 



This is, at least, a very candid admission, because the whole doctrine of the 

 descent of the sap is still far from being clearly established. It has, indeed, 

 been declared by persons of undoubted veracity, that the counter currents of the 

 crude and elaborated sap may be seen in the almost transparent petioles of 

 certain leaves. Thus much we are bound to believe ; but, as our keenest 

 observation carries us no further than the base of the petiole, we can only 

 imagine its subsequent distribution, having no means of proving whether it 

 retires by the bark or by the alburnum ; more especially as we know that the 

 richest sap is always found by tapping, not at the bottom, but at the top of the 

 tree. 



Speaking of endogens, particularly palms, he says, truly, that the stems are 

 formed by the persisting bases of the leaves; but he has been misled by 

 others into a belief that the youngest leaves are formed at the top of the stem, 

 and that the woody bundle which connect them with the system " passes 

 downwards in the softest part of the stem, which is its interior ; but after pro- 

 ceeding for some distance in this manner it turns outward, and interlaces itself 

 with those which were previously formed." Unfortunately for our author's 

 teachers, the very reverse of this is the fact : the fibrous constituents of each 

 leaf or frond originate at different depths in the interior of the stem, whence 



